Critical factors for implementing sludge processing of the Rome Wastewater Treatment Plants

The use of sludge thermal drying after mechanical dewatering implies a sharp increase in capital costs but can be effective in reducing both transport and disposal costs. In this paper the most critical factors affecting economics in implementing with sludge thermal drying the four wastewater treatment plants serving the city of Rome are discussed. As alternatives, replacement of belt presses with filter presses in the plant of Rome East, due to the low concentration of sludge cake (18%) presently produced, and centralised treatment in the plant of Rome South of the sludge produced in this plant and in the plant of Rome Ostia are also considered. Thermal drying appeared more convenient especially when dewatering operation is poorly performed (Rome East), but in the large plants (Rome South alone or with Ostia) the upgrading of the plant is always economical. Other contingent and local situations can also play an important role in this prospect (distance from the landfill site, transport and disposal unit costs), thus making not convenient the adoption of thermal drying for the plants of Rome North and Ostia. For disposal cost lower than 52 ECU/t replacement of the present belt presses with filter presses in Rome East plant is more advantageous than the construction of a drying plant.